1. Field of the Invention
The present invention relates to an image reading apparatus and a method for adjusting the same, and more particularly to an image reading apparatus that corrects MTF characteristics of at least one of image data read from a first surface and a second surface of a double-sided original, and a method for adjusting the same.
2. Description of the Related Art
Conventionally, as an image reading apparatus for use in a copying machine or the like, there has been proposed an image reading apparatus which performs so-called “moving original reading” (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2001-285595). In the image reading apparatus, originals are conveyed by an original conveying device, page by page, onto an original platen glass, and the original is exposed to light by an exposure device fixed in facing relation to a conveying passage so as to read an image on each original.
Further, conventionally, an image reading apparatus has been proposed which is provided with two image readers so as to read the front and back surfaces of the original only by one original conveyance, for improved original reading speed (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2004-187144).
Image reading apparatuses of the above-mentioned kind include an image reading apparatus which is provided with a reducing optical system using a CCD (Charge Coupled Device) line sensor and a reducing lens, and a unity magnification optical system using a CIS (Contact Image Sensor). Such an image reading apparatus suffers from differences in density characteristics and MTF (Modulation Transfer Function) characteristics occurring between the front and back surfaces of an original. When a pattern of stripes of light and shade is formed through an optical system, a contrast between light and shade stripes in the pattern is degraded by the diffraction and aberration of the optical system. The term “MTF characteristics” is intended to mean the degree of degradation of the contrast in the pattern of light and shade stripes. The contrast is more largely degraded as the frequency (spatial frequency) of the pattern becomes higher.
Further, in the above-described image reading apparatus, when an original contains a mixture of images formed by different density reproduction methods, such as continuous images and halftone-dotted images, small differences in the MTF characteristics make the differences in density conspicuous.
Furthermore, also when optical systems of the same type are used for reading the front and back surfaces of an original, respectively, there similarly occur different reading characteristics between the front and back surfaces of the original due to different assemblies of component parts of the optical systems, different spectral characteristics of the same, etc.
Out of the differences in reading characteristics, the differences in the MTF characteristics have conventionally been reduced by an image reading apparatus that performs adjustment of a reading optical system, or MTF filtering or smoothing filtering on image data read from an original (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2001-157052 (Patent No. 3660180)). This image reading apparatus reduces variation in the MTF characteristics in the same plane of the original, and the differences in the MTF characteristics between the front and back surfaces of the original.
However, the conventional image reading apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2001-157052 (Patent No. 3660180) is assumed to uniformly perform MTF filtering or smoothing filtering in the same plane of the original.
Even when the MTF filtering is uniformly performed in a plane, if there is a large variation in the MTF characteristics in the plane, e.g. if there is a large difference in the MTF characteristics between an end of the original and a central portion thereof, it is sometimes impossible to obtain desired results from the MTF filtering.
Further, even when an MTF filter is changed between reading modes, such as a pressure plate reading mode (fixed original reading mode) and a moving original reading mode, if the MTF characteristics largely vary in the same plane or between the front and back surfaces of the original, it is difficult to match the MTF characteristics precisely between the front and back surfaces of the original or reduce variation in the MTF characteristics in the same plane, for the same reason described above.
Therefore, the prior art suffers from a limited reduction of occurrence of moire in the same plane or a limited reduction of the color differences between the front and back surfaces of an original.